3 Explosions Rock Cairo, Killing 5

Egyptian security officers and firemen stand at the scene of a bomb attack that reportedly targeted an Egyptian police car parked by a metro station in the district of Dokki on January 24, 2013 in Cairo, Egypt. (Photo by Ed Giles/Getty Images).

By Sarah Lynch, Special for USA TODAY

CAIRO - A massive explosion hit Egyptian police headquarters in downtown Cairo early Friday in the deadliest bomb blast in Egypt's capital since a military coup ousted Islamist president Mohamed Morsi last summer.

A car bomb tore through the site around dawn, killing 4 and wounding 76, according to the state new agency MENA.

Strips of the police headquarters' façade were damaged in the blast, which also blew out the building's windows. After the explosion, a large plume of black smoke billowed above the capital, pictures showed.

In a separate, smaller attack, a home-made bomb exploded across the Nile River in Dokki, the state news agency said. One was killed and eight were wounded in the attack, which targeted police vehicles, the Associated Press reported.

A third bomb went off near a police station in Giza, the state news agency reported. No casualties were immediately reported.

The blasts come a day before the three-year anniversary of the uprising against Hosni Mubarak, president of Egypt for three decades until hundreds of thousands took to the streets to protest against him in early 2011. Six months ago, Muslim Brotherhood figure Mohamed Morsi was the second Egyptian leader to be ousted in 2 ½ years.

Cairo's U.S. Embassy has condemned "in the strongest possible terms the heinous terrorist attacks this morning in Cairo."

"We fully support the Egyptian government's efforts to bring the perpetrators of these crimes to justice," the embassy said in a statement on its website. "The Embassy extends its deepest condolences to the families and friends of the victims, and we hope for the quick and full recovery of the injured."

In a speech delivered at Cairo's Police Academy earlier this week, before Friday's blast, Egypt's interim president Adly Mansour said celebration of the January 25 revolution "has been marred by the ugliness of terrorism."

"I assure you that just as we succeeded in the 1990s to eradicate terrorism we will again wipe it out," he said, according to a presidential statement. "Failure in our current battle is not an option. God willing, together, aided by the resolute unity of the sons of our nation and working hand in hand with law enforcement we will expel this danger from our beloved homeland."

Several blasts have rocked Cairo since Morsi was overthrown last August by Egypt's army chief after millions of Egyptians protested his leadership.

A car bomb targeted Egypt's interior minister last summer, although he was unharmed. Last week, an explosion damaged a court complex in North Giza just before polls opened in a vote on a proposed new constitution.

Attacks have occurred elsewhere across the country. In December, at least 16 people were killed in an explosion at a police headquarters in Mansoura, in Egypt's Nile Delta region. Ansar Beit al-Maqdis, an al-Qaeda inspired group, claimed responsibility for the attack, local news media reported.

Egypt's Sinai Peninsula, which shares a border with Israel and the Gaza Strip, has been home to regular attacks against security and government outposts amid growing militant activity in the restive region.

In recent weeks, Egyptian authorities escalated a broadening crackdown on opposition, claiming it is trying to fight terrorism. In December, Egyptian authorities declared the Muslim Brotherhood a terrorist group after imprisoning many of its members and leaders.

The Brotherhood has repeatedly denied that it has used violence in its opposition to the government, which it claims took power by force last August and is therefore illegitimate.

"The National Pro-Legitimacy Alliance condemns the #CairoBomb & reaffirms commitment to peaceful struggle against the coup," a Brotherhood-led, anti-coup group said on Twitter.

On Saturday, anti-government protests and rival celebrations marking the 2011 uprising against Mubarak are expected to take place across the capital amid heightened security, raising prospects for clashes and more violence.

Ahead of the weekend's demonstrations, the U.S. Embassy in Cairo warned citizens to remain alert and review plans for personal security.

"U.S. citizens are advised to elevate their level of awareness as we move into the weekend and limit their movements on Saturday, January 25 to the near vicinity of their neighborhoods," a security message for U.S. citizens said.